The last time I prepared for a vacation, friends advised that I should reserve my airline tickets on the Internet. "You'll save money that way," they were sure.

Frontier Airlines had the lowest fare to my destination online. But before signing off on it, I played a hunch and called Frontier for its person-to-person fare.

The clerk named a fare $15 less than the Internet tariff. I recalled the experience when I planned this year's trip. Once again, I saved $15.

The story illustrates the divergent prices for the same product or service that consumers can experience, depending on how and when they approach the company.

The conventional wisdom that buying online is cheaper than a person-to-person contact is worth challenging.

And prices can change quickly. The Frontier clerk asked whether I needed a rental car. He said that Hertz had an agreement with Frontier for a 20 percent discount and that he could connect me with Hertz.

When the Hertz clerk quoted me a price, I asked whether that included the 20 percent discount.

"No," she said. "We only give a 10 percent discount."

I called back the next day and got another voice. I repeated that I was a Frontier customer and asked about the airfare cost. She cited the same figure her colleague had the day before.

But when I asked about the rental car discount, she said it was 5 percent. I said I had been told by Hertz the day before that it was 10 percent.

"Oh, the discount changes every day," she said.

Full disclosure

Norman Larson of Ellsworth, Wis., is a retired University of St. Thomas journalism professor. He says he searched the Internet for a hotel in Burnsville and found HotelsByCity.com, where he spotted this ad: "Burnsville Hotels, 110% low rate guaranteed."

Larson said, "My credit card was charged $84.24. ... But when I checked out, the receipt was $25 less than what I had already paid."

So Larson went back to the HotelsByCity.com site for an explanation. Included in a long response was this:

Buyer beware

"Facilitation fee: We act as an intermediary between travel suppliers and consumers for the booking of a payment for travel accommodations. To facilitate the provision of travel accommodations to consumers, we enter into agreements with various travel suppliers which govern the booking of and payment for such accommodations through this website. The price listed for travel accommodations includes the amount(s) paid to the relevant travel supplier(s) in connection with facilitating your travel arrangements, as well as a service fee for facilitating such travel arrangements."

Translation: We get a commission paid for by the consumer.

The lesson is this: Whether you're shopping in person or on the Internet, "buyer beware" is still the best policy.